1. Field of the Invention
The present invention relates to vesicular photography, and more particularly, to improved vesicular matrices for use in vesicular photography, and novel copolymers for use therein.
2. Description of the Prior Art
Diazo type photoreproduction is of two different types. Each is based on the light sensitivity of aromatic diazonium salts, and the fact that such salts undergo decomposition by light can be used in two ways: (1) decomposition, in which nitrogen evolves as gas in exposed areas; and (2) "coupling", wherein the diazonium compound retained in not exposed areas reacts with certain couplable color-forming components, i.e., a "coupler" or "azo-coupling component", to effect formation of an azo dye species.
The present invention is concerned with the former type of reaction, that is, the vesicular process. Vesicular images are formed in a photographic film by small bubbles or vesicles of gas which are formed and trapped in the areas of the film exposed to light and which scatter light. Vesicular film has a colloid or a resin coating or vehicle on a backing material and a light sensitive agent or sensitizer, such as a diazonium compound, dispersed throughout the coating. When the film is exposed to light, the sensitizer releases molecules of a gas. In the case of diazonium compounds, the gas is nitrogen. Rather than forming vesicles immediately, the vesicles are formed when the film is heated, presumably because the vehicle is relaxed sufficiently on heating for the gas molecules to form bubbles, and for the bubbles to expand. The formation of the vesicles makes the vehicle opaque to transmission of light in the exposed areas, and the vesicles also reflect and scatter light so that they appear white.
Preservation of the image depends upon the vehicle maintaining its rigidity and the vesicles being fixed in place. Although the rigidity of the vehicle is reduced during development to permit formation of vesicles and to allow the vesicles to expand, the rigidity is restored by cooling to give permanency to the image after development. For permanency, the vehicle must remain rigid under the heat and moisture conditions to which it will be exposed.
The first vesicular materials employed gelatin as the vehicle. However, such images faded rapidly because of the sensitivity of gelatin to water. Gelatin vehicles absorbed moisture from the atmosphere and became soft, allowing the vesicle to collapse, thereby destroying the image.
Numerous patents describe later attempts at developing suitable matrices. Although numerous such matrices have been developed, only three systems are currently being employed commercially; those being systems based on saran, polyhydroxyether, and poly-alpha-chloroacrylonitrile.
One material which appeared promising but has never been commercialized is polymethacrylonitrile. Despite a suitable softening point and a suitably low diffusion constant for nitrogen, its potential has yet to be realized.
The basic patent concerning polymethacrylonitrile is U.S. Pat. No. 3,161,511 to Parker and Mokler which describes a vehicle manufactured using polymethacrylonitrile as the resin matrix. As stated in that patent, homopolymers are preferable to copolymers because they are easier to manufacture.
The difficulty in the manufacture of copolymers is controlling proportions when there are two or more monomers. Monomers typically do not polymerize at the same rate, i.e. if two monomers are polymerized together, one will enter into the reaction more easily than the other. As the reaction proceeds, the more reactive monomer will be consumed more rapidly, and the relative proportions of the two monomers will change. Since the rate at which the monomers enter into the reaction depends on their relative proportions as well as their inherent activity toward the reaction, their reaction rates change. As the reaction proceeds, the relative proportion of the monomers entering the growing polymer changes, and the polymer produced at the beginning of the reaction has different monomer proportions from the polymer produced later.
To avoid this result, at least in part, monomer is sometimes continuously added during the reaction to maintain constant proportions. Another possibility is to accept variations in the proportions of the monomers in the polymer and to later thoroughly blend the resulting polymers to assure uniform properties throughout each batch. The average properties then meet the needs of the product. However, this may result in variations from batch to batch because of variations in the completeness of the reaction or other conditions.
U.S. Pat. Nos. 3,622,335 and 3,622,336 to Notley are improvements on the Parker and Mokler patent mentioned above. These patents represent attempts to produce copolymers in spite of the problems discussed in the Parker and Mokler patent. The '335 patent employs copolymers of alpha substituted acrylonitrile (which includes methacrylonitrile) and a styrene-type monomer. It is stated therein that the proportion of comonomer must exceed 5 mole percent and it is not generally desired to exceed 60 mole percent or the desired characteristics of the substituted acrylonitrile will be significantly reduced. Furthermore, it is stated that any two copolymers produced therein can be blended where they are compatible in a common solvent or mixed solvent, and that the essential polymer can be blended with limited amounts of a non-essential but compatible polymer such as cellulose acetate, cellulose acetate butyrate, polyalphamethylstyrene, polyvinylidene chloride, acrylonitrile copolymer and polymethylmethacrylate.
The '336 patent is similar to the '335 patent except that it describes a copolymer of alpha-chloroacrylonitrile and alpha-methacrylonitrile. The ratio of the monomers is stated to be between 1:4 and 4:1, and a preferred ratio is stated to be 1:2.
U.S. Pat. No. 3,661,589 to Notley describes a different approach. Rather than copolymerization, a vesicular imaging film was formed by mutually dispersing two resin solutions at the threshold of compatibility but having a common or mutual solvent, each of the resin solutions containing a sensitizer which liberates gas on irradiation, coating the resulting dispersion as a thin film, and then drying. The resulting thermoplastic film is stated to be an intimate dispersion of one hydrophobic resin in the other hydrophobic resin with the sensitizer dispersed throughout.
The Notley '589 patent also sets forth the criteria that a hydrophobic resin used in vesicular photography must satisfy, and reiterates that these criteria are very comprehensive and quite critical. These include very low permeability, good rigidity under ambient conditions, a convenient softening temperature at which the polymer is sufficiently fluid to permit vesicles to form but at which the gas permeability is still not excessively high, good solubility, good film forming characteristics, good adhesion to inert substrates and good binding for high concentrations of sensitizer.
In the structure described in the Notley '589 patent, hydrophobic resin is encapsulated within a continuous coating of another hydrophobic resin, with the light sensitive gas generating material dispersed throughout the encapsulate and the encapsulating resin. The optimum amount of encapsulated resin is stated to exceed 5% but generally not to exceed 50% of the total resin. The threshold incompatibility described therein is illustrated by mixing two parts of a 20% solution of polystyrene in butanone with one part of a 20% solution of a polyvinylidene chloride/acrylonitrile copolymer in the same solvent. By using polystyrene to saran in a ratio between 1 to 1 and 1 to 6, the solution is only slightly hazy and good coating quality is said to be achieved from the agitated solution. The threshold incompatibility is seen when the dispersion is allowed to stand since it separated into two layers, one rich in polystyrene and the other in saran. Preferred encapsulating resins therein are saran, polyvinylacetals, copolymers of methacrylonitrile, and chloroacrylonitrile homopolymer and copolymers. The choice of encapsulated resin is based on the diffusion coefficient and the refractive index. Encapsulating/encapsulated resin combinations listed include saran with polystyrene, ortho/para polychlorostyrene or cellulose acetate; formaldehyde polyvinyl acetal with polystyrene or cellulose acetate; formaldehyde polyvinyl acetal with polystyrene or polyketone; methacrylonitrile-methylmethacrylate copolymer with saran or cellulose acetate; and chloroacrylonitrile-styrene copolymer with polystyrene.
While polymethacrylonitrile has a suitable softening point (Tg=120.degree. C.) which should supply good developing and good thermal stability of vesicles, and has a diffusion coefficient for nitrogen which is also very low (approximately 5.times.10.sup.-13) which should prevent nitrogen from escaping prior to the development of the film, the technology described by the above patents is not currently being used commercially. Such compositions are slow in comparison to presently marketed films, with the problem appearing to be related to insufficient nucleation, possibly because of high lattice homogeneity of the polymer. For example, in comparison to a similar polymer (poly-alpha-chloroacrylonitrile), a polymethacrylonitrile film is much slower.
Accordingly, a need exists for a vesicular vehicle having the desirable properties of polymethacrylonitrile with a speed comparable to those of commercially available materials.